Cooling fans are widely used in electronic devices to effectively dissipate heat sources in the electronic devices into ambient air outside the electronic devices, so that the electronic devices can maintain normal operation. In a conventional multi-fan speed output structure, the rotation speeds of all the fans are exactly output to a system end and the system end performs a logical operation on these fan speeds. Therefore, the system end has an increased operation load, which tends to cause delay in the operation of the whole system and increased heat production in the system. It is therefore desirable to work out a way to reduce the system's operation load and accordingly reduce the heat energy produced in the system.